This is the 9033rd most frequent Japanese word.
海底
"Seafloor" or "seabed."
In this sentence, "海底" is used to refer to the physical seafloor where the shipwreck is located.
海底に沈む船を探索する。
Exploring the sunken ship on the seafloor.
Here, "海底" denotes the ocean floor as a geographical feature studied by scientists.
科学者たちは海底の地形を調査している。
Scientists are studying the terrain of the ocean floor.
In this case, "海底" refers to the concept of 'undersea' as part of the tunnel's description.